U.S. Pat. No. 4,612,162 to Morgan et al., the teachings of which are hereby incorporated by reference, describes a method of forming high density metal articles by hot isostatic pressing and sintering a metal powder.
Tungsten parts may be made by powder metallurgical techniques. Tungsten-based metal powders may be pressed isostatically and then sintered to achieve a density equivalent to 92%-97% of the theoretical density of the metal. However, tungsten is an extremely hard, brittle material and is thus very difficult to machine. It is susceptible to breaking, chipping and distortion under most machining operations. In particular, it has previously been impossible to fabricate threads in large tungsten parts because of the extreme brittleness of the tungsten material. "Large" parts, as the term is used herein, are those parts having a diameter or thickness greater than two inches.
Pressed and sintered tungsten metal parts may be more easily machined, and even threaded, if they are first mechanically worked to increase the density of the parts. However, mechanical working is only effective on relatively small tungsten parts, that is, on parts having a diameter or thickness less than two inches. Large tungsten parts cannot be uniformly densified through mechanical deformation due to density gradients throughout the part and size and power limitations of the working equipment.